Heating Ventilating and Air Conditioning (HVAC) systems are well known. These systems typically use forced air to heat, cool and ventilate an area, zone or room. A heater and a cooler is provided which can heat or cool or just circulate the air, and the air is sent to each zone by means of damper controlled ducts and a fan.
U.S. Pat. 4,530,395 as well as 4,931,948 to Parker et al describes a single zone HVAC controlled for operation in multiple zone arrangement utilizing a variable volume air distribution method. In this system there is need for computerized thermostats in addition to the central control system 10 for the HVAC equipment In addition, the modulating dampers are directly controlled by the computerized thermostats. This system bases its control by voting on which mode of energy or type of air to supply to a zone. Many such systems base the control of the damper by the thermostat directly and therefore require a delivered air temperature sensor in the damper to decide whether to open or close the damper. The voting is then done by feeding the thermostat information back to the central control equipment. Such systems are usually very costly and can still starve a minority zone.
U.S. Pat. No. 5,092,394 to Foster and assigned to the present assignee, describes a switching panel and system for controlling zone heating and cooling systems. This reference only controls a manually selected mode, either the heating or the cooling. The system cannot satisfy a zone calling for cooling while the system is in the heat mode even if there is no present heat call or visa-versa.
Most of the prior art systems require a manual mode change to determine if they will control either the heating or the cooling, but cannot operate sequentially using automatic changeover based on the zone requirement. There are some other systems which automatically change to the other mode based on an outdoor or building-wide sensing device. However, they are based on separate thermostats, such as a thermostat in the system return air duct, or outside of the building. There are also some systems which use a voting or counting method of the individual zones. However, none of these systems can satisfy each of the zones in turn, when there are conflicting calls, nor do they provide the equipment protection in the simple manner to be described.
Many patents, such as U.S. Pat. No. 4,997,030, assigned to Toshiba, describe the control of a variable air volume system which is based on the principle of allowing air into a zone via a modulating damper controlled duct when the zone is above or below the set point which matches the temperature of the delivered air. It then limits the amount of air until the damper is closed when the zone temperature exceeds the differential. This method assumes that the duct is the source of the energy and that the zone will only dissipate that energy. If the source of the energy is within the zone such as many electrical appliances or enters via another route, such as sunny windows, the control system does not always provide a comfort range control.
U.S. Pat. No. 4,897,798 assigned to AT&T, attempts to adapt the set point of the system based on the thermal properties of the building.
In each case there is an attempt to emulate an individual set of equipment for each zone. The purpose of the system to be described, is to allow each zone to demand its own comfort range while insuring that no zone is ignored.